Wikipedia has this to say about "zero day exploits": A zero-day (or zero-hour or day zero) attack or threat is a computer threat that tries to exploit computer application vulnerabilities that are unknown to others or undisclosed to the software developer. Zero-day exploits (actual code that can use a security hole to carry out an attack) are used or shared by attackers before the software developer knows about the vulnerability.

I wish every exploit could just be called an exploit (sans "zero day" in front of everything) unless it's specifically 1) a vulnerability the company has chosen not to fix, or 2) a vulnerability some guy somewhere knew about but hadn't used in order to keep it valuable. It's like if we were to start calling Microsoft Office "Microsoft Office for Windows" incessantly. It's assumed, unless you're specifically on a Mac or running it in WINE or something.